1. Field of the Invention
The present invention relates to the field of semiconductor substrate processing equipment. More particularly, this invention relates to a purge ring and associated fastening device.
2. Background of the Related Art
Chemical vapor deposition ("CVD") is one method used in semiconductor fabrication to deposit thin films on a substrate. CVD delivers precursor gases to a vacuum chamber and onto a substrate where the gases react and form a thin film thereon. One problem with CVD of material to the top surface of a substrate is that material also deposits on the edge and backside surfaces of the substrate. Because the edge and backside surfaces of a substrate are typically rougher than the highly polished top surface, material deposited on these surfaces tends to flake off creating particles within the system that can damage the chamber and substrates processed therein. In addition, material deposited on these surfaces may damage the integrity of a substrate by causing the substrate to adhere to the support member.
Purge gas has been used to prevent edge and backside deposition by delivering the purge gas to the edge of a substrate typically through a support member. For example, a purge ring is positioned on the support member adjacent to the periphery of the substrate. The purge ring forms an annular channel with the support member which directs the purge gas toward the edge of the substrate. The purge gas exerts a positive pressure near the perimeter portion of the substrate and reduces the likelihood that processing gases will reach the edge and backside surfaces of the substrate.
The purge ring can be fixedly attached to the support member by such methods as welding, bolting, brazing, or screwing. However, because material deposits on the purge ring and in the annular channel, removable purge rings are preferred to allow the purge ring to be easily and quickly cleaned or replaced. Purge rings are commonly gravitationally or frictionally held to the support member. Because removable purge rings are not fixedly attached to the support member, the purge gas or other process conditions may cause vertical or rotational displacement of the purge ring which can result in edge/backside deposition, particle generation, and damage to the edge of the substrate.
Movement of the purge ring is a significant problem when the purge ring is used with a ceramic pedestal. Ceramic pedestals are used in high temperature applications because aluminum pedestals can corrode, backsputter, and warp at high temperatures. However, a problem with ceramic is that it is more brittle than metal and more likely to crack. Thus, movement of the purge ring can damage the ceramic pedestal and can create particles within the system. In addition, the purge ring cannot be attached to the ceramic pedestal with common joining devices, such as welding, bolting, brazing, and screwing, because of the likelihood of cracking the ceramic components and the difficulty associated with working with ceramic components.
Therefore, there is a need for a purge ring and removable assembly which restricts vertical and rotational movement of the purge ring without damaging the pedestal or other system components.